Shigella species, the causative agents of bacillary dysentery, are unusually infectious. Volunteer studies have shown that as few as 10 ingested organisms are sufficient to produce disease. This is particularly remarkable when one considers that the inoculum must pass through the stomach, with a pH of less than 3.0 in order to reach the intestines where entry and replication within colonic epithelial cells results in dysentery. Enteric pathogens such as Salmonella species and Vibrio cholera require an infectious dose over 10,000-fold greater in order to cause enteric disease. Shigella is able to survive exposure to pH 2.5 for several hours whereas Salmonella species are extremely acid sensitive. Sensitivity of acid is felt to be major determinant of infective dose for these organisms. Avirulent Salmonella species are being widely exploited as potential hosts for the development of oral vaccines based on a number of different antigens because of their ability to elicit cell mediated immunity. The acid sensitivity of Salmonella species is an impediment to their usefulness in vaccine construction. The objectives of this study are to characterize the genetic and physiologic basis of acid resistance on an acid resistance in Shigella species and to utilize this information to construct more efficacious oral vaccine strains. We have identified a gene from Shigella flexneri, rpo(s), which confers acid resistance on an acid sensitive Escherichia coli, HB101. Rpo(s) encodes a stationary phase sigma factor which acts indirectly in conferring acid resistance on Shigella species. We have recently obtained evidence that this gene is also required for the expression of acid resistance in enterohemorrhagic E. coli, an organism which may also have a low infective dose. In order to identify specific genes required for acid resistance, transportation mutagenesis was used to construct acid sensitive mutants of S. flexneri. Two of these insertions fall in known loci, nadB and the mar operon. Two more insertions fall into genes encoding secreted proteins for which no known homologies were found. The expression of some of these genes appears to be regulated by rpo(s). The identification and further analysis of this set of acid resistant genes will be useful in the construction of acid resistant oral vaccine strains as well as for determining the role rpo(s) regulated genes play in virulence.